In an optical communication field or an optical measurement field, a waveguide type optical modulator in which an optical waveguide and a modulation electrode are formed on a substrate having an electro-optical effect has been often used conventionally.
In particular, since the amount of transmitted information tends to increase with the development of multimedia, it is necessary to widen a band of a light modulation frequency. In order to realize that described above, an external modulation method using a LiNbO3 (hereinafter, referred to as ‘LN’) modulator or the like has been used. However, it is necessary to realize speed matching between a light wave and a microwave, which is a modulated signal, and to reduce a driving voltage in order to widen the band of the LN modulator.
As a means for solving the problems, it is known that a condition of speed matching between a microwave and a light wave is satisfied, and at the same time, a driving voltage is reduced by making a substrate thin.
In the following Patent Document 1 or 2, an effective refractive index of a microwave is reduced by providing an optical waveguide and an modulation electrodes in a thin substrate (hereinafter, referred to as a ‘first substrate’) having a thickness of 30 μm or less and bonding another substrate (hereinafter, referred to as a ‘second substrate’) having a dielectric constant lower than the first substrate to the first substrate, such that the speed matching between the microwave and a light wave is realized and the mechanical strength of the substrate is raised.
Patent Document 1: JP-A-64-18121
Patent Document 2: JP-A-2003-215519
In Patent Document 1 or 2, LN is used for the first substrate and a material having a lower dielectric constant than LN, such as quartz, glass, and alumina, is used for the second substrate. In the combination of these materials, DC drift or temperature drift according to a temperature change occurs due to a difference between coefficients of linear expansion. In order to eliminate such problem, Patent Document 2 discloses that the first substrate and the second substrate are bonded to each other using an adhesive having a coefficient of linear expansion close to the first substrate.
However, in the case of an optical control element where an optical waveguide is formed, for example, a Mach-Zehnder type LN optical modulator, as shown in FIG. 1 (a), a problem occurs in that input light 10 not coupled with an optical waveguide within the optical modulator propagates through a substrate other than the optical waveguide as decoupled light in a part where an optical fiber and the optical modulator are combined, or scattered light 11 or radiant light 12 in the optical waveguide or particularly in a Y-branch part propagates through the substrate in the same manner. Further, as shown in FIG. 1(b), there also occurs a problem, such as crosstalk in which a part 13 of propagating light shifts to another optical waveguide, between adjacent optical waveguides such as branched optical waveguides.
Such decoupled light, scattered light, and crosstalk light (hereinafter, referred to as ‘non-guided light’) are incident on the optical waveguide. This causes a trouble, for example, a modulation curve (ideally, a function of cos2θ) of the optical modulator is distorted.
The inventors have found out that the following phenomena are especially noticeable particularly in the case when the thickness of a substrate formed with an optical waveguide is 30 μm or less or twice the mode field diameter of guided light or less.
(1) The mode diameter of guided light tends to extend in the lateral direction (direction parallel to a substrate surface) as compared with the longitudinal direction (direction perpendicular to the substrate surface), and so decoupled light or various kinds of scattered light increase, and crosstalk between waveguides increases.
(2) Non-guided light, such as decoupled light, propagates through the substrate like guided light and is recoupled with a later-stage waveguide.
Due to the phenomena described above, a modulation curve is largely distorted. As a result, a serious problem in characteristics of an optical modulator or control of the optical modulator occurs, for example, the extinction ratio of the light modulator deteriorates or the maximum amount of transmitted light of the modulation curve differs.
An effect in the case when the substrate is made thin will be described using a case of an optical modulator, which has a Mach-Zehnder type optical waveguide shown in FIG. 2(a), as an example. FIGS. 2(b) and 2(c) are cross-sectional views taken along the dashed-dotted lines A and B, respectively. The cross-sectional shape 23 of light waves passing through two branched optical waveguide portions 3 and 4 is a shape extending in the lateral direction of a substrate 1, as shown in FIG. 2(b). In addition, a light wave 12 radiated from a Y-branch part where branched optical waveguide portions join also shows a shape extending in the lateral direction as shown in FIG. 2(c), and the light wave 12 is extremely close to a light wave 24 propagating through an optical waveguide 5. In such condition, non-guided light, which is the radiant light 12, and the light wave 24 propagating through an optical waveguide 5 are easily recoupled with each other, and it is a main cause of deterioration of a modulation characteristic of the optical modulator. Moreover, although not explained in FIG. 2(a), reference numerals 21, 22, and 20 denote a modulation electrode, a ground electrode, and an adhesive layer for bonding the substrate 1 and a reinforcing plate 21 to each other, respectively.
In order to clarify the influence of a change in the thickness of a substrate, a change in a degree of flatness of the mode diameter of an optical waveguide at the time of changing the thickness of a substrate is shown in FIG. 3. In FIG. 3, a case is assumed in which a dielectric (refractive index n=1.45) is disposed below an LN substrate, an air layer is disposed above the LN substrate, the waveguide width depending on thermal diffusion of Ti is 6 μm, and the Ti thickness at the time of film formation is 500 Å or 900 Å. Assuming that the diameter in the lateral direction is ‘x’ and the diameter in the longitudinal direction is ‘y’, ‘x/y’ is expressed as a degree of flatness of the mode diameter of the optical waveguide. In this case, it is understood that the degree of flatness changes abruptly when the thickness of the LN substrate reaches 30 μm or less in the case that the Ti thickness is 500 Å and when the thickness of the LN substrate reaches 15 μm or less in the case that the Ti thickness is 900 Å.